Many species of bacteria swim by means of flagella, which are thin helical filaments turned by rotary motors in the cell membrane. Flagellar motility is a factor in the virulence of many human pathogens, including those that cause ulcers, syphilis, burn wound infections, and some diarrhea. The flagellar motor obtains energy for rotation from the membrane gradient of protons or, in some species, sodium ions. The molecular mechanism of rotation is not understood. Rotation must be driven by forces generated between the rotor (the rotating part) and the stator (the non-rotating part). The stator is formed from the integral membrane proteins MotA and MotB, which function to conduct ions across the membrane and to couple this ion flow to rotation. Each motor contains several MotA/MotB complexes, which function independently to generate torque. Mutational and physiological approaches have been used to identify functionally important amino acid residues in MotA and MotB, and to show that protons flowing through the motor interact with a particular aspartic acid residue(Asp32 of MotB) to drive conformational changes in the MotA/MotB complexes. Here, biochemical and structural studies are proposed that will provide a detailed picture of the conformational change that serves as the "power stroke" in the motor, and a framework for understanding the mechanism of rotation.